A typical telescopic system collects light through an entrance aperture, passes the light through reflective optics and onto a plane for viewing. The light impinges upon a primary mirror and is reflected toward a secondary mirror. The light can then be reflected from the secondary mirror toward a focus and extends through an aperture in the primary mirror to a series of mirrors aft of the primary mirror. The light can be reflected by these mirrors out of the system to a camera or detector.
It is often desirable to scan such a system across a wide field of view for pointing or scanning in a raster or rectilinear method. When scanning, the output image must be directed onto the detector to allow for continuous viewing, recording or analysis. To that end, the entire system, including the mirrors aft of the primary mirror and the detector, rotates as a unit. As the entire system rotates, a large volume is necessary to permit rotation. This can limit the usefulness of the system in the confined enclosures of some airborn and space applications. Also, mechanical and/or optical interference inherent in such systems limit the angular range through which the system can be rotated.
A continuing need exists however for a more compact scanning system that provides accurate imaging at long focal lengths and over a broad range of wavelengths.